1. FIELD OF THE INVENTION
This invention relates to a relatively simple and low-cost circuit configuration that implements a capacitance-to-voltage converter which is highly immune to stray capacitance and which accurately measures the unknown capacitance of a capacitor.
2. PRIOR ART
Existing capacitive-type sensors are known which are characterized by considerable stray capacitance-to-ground as a consequence of bonding wires, connectors, headers, ground shields, etc. For example, capacitive-type pressure transducers sense pressure by undergoing a change in dimension, whereby capacitance is a function of applied pressure. The change in capacitance for a full scale pressure change may be only a few picofarads. This small change is difficult to measure in the presence of high stray capacitance-to-ground which may vary with temperature, shock and vibration. More particularly, sophisticated capacitor bridges, RC and LC oscillator-type circuits, and RC charge and discharge time-based circuits are either sensitive to stray capacitance or require highly accurate standard capacitors, such that the cost of the measurement equipment or circuitry may far exceed the cost of the sensor.
Occasionally, it is desirable to measure small capacitors, such as integrated circuit MOS capacitors. Due to the small size of these capacitors, it is difficult to accurately measure the capacitance values thereof while using conventional measurement techniques. Morever, the stray capacitance of the test fixtures also interferes with obtaining a precise measurement of capacitance. On-chip integrated capacitance-to-voltage conversion circuits are available which attempt to reduce the effect of stray capacitance and provide a more accurate measurement of such small capacitors. These conversion circuits use switched capacitor techniques that generate a voltage or digital work which is proportional to capacitance or the difference between a reference capacitor and an unknown capacitor. However, the accuracy of these conversion circuits is degraded by clock feedthrough and nonlinearities. Moreover, such circuits are also characterized by limited resolution.
Precise measurement of capacitance is usually accomplished by comparing an unknown value of capacitance with a known value using a bridge circuit. Extremely accurate and high resolution (to 1 ppm) shielded capacitor bridges have been constructed and are used in metrology laboratories. However, a high degree of skill is required to operate these instruments in order to achieve accurate results on small capacitors, since the measurement is greatly influenced by the stray capacitance of conventional test fixtures.
Other capacitance measurement instruments depend upon accurately measuring the charge and discharge times of RC circuits. Such instruments are popular because of their simplicity and compatibility with the single chip digital voltmeter integrated circuit. Unfortunately, such instruments have limited resolution and are very sensitive to stray capacitances of test fixtures or test leads.
Still other techniques indicate capacitance by measuring the impedance of the capacitor using a 4-terminal pair configuration which reduces the effects of stray capacitance. Instruments based upon this technique calculate capacitance from a measurement of reactance. Therefore, such instruments, while being very accurate, are also very complex and expensive.